1. Field of the Invention
The present invention relates to a dichroic mirror and a projector having the same.
2. Related Background Art
A conventional dichroic mirror is shown in a schematic view of FIG. 1. A detailed arrangement of this projector is described in Japanese Laid-Open Patent Application No. 61-13885.
As shown in FIG. 1, a polarizing beam splitter 83 is arranged on the output side of white light emitted from a light source 82. First and second dichroic mirrors 84 and 85 are formed at bonding surfaces each obtained by bonding two rectangular prisms and are sequentially arranged in a direction (left direction) opposite to that of light reflected by the polarizing beam splitter 83. The first and second dichroic mirror 84 and 85 separate the white light into red light, green light, and blue light, radiate these light beams on first to third reflection liquid crystal panels 81R, 81G, and 81B for forming red, green, and blue images, respectively, and synthesize image light beams of the respective colors emerging from the liquid crystal panels 81R, 81G, and 81B. The image light beams synthesized by the first and second dichroic mirrors 84 and 85 are enlarged and projected on a screen (not shown) through a projection lens 86.
The liquid crystal panels 81R, 81G, and 81B often comprise ECB (Electrically Controlled Birefringence) panels each having characteristics for partially rotating a polarization plane of incident light (S-polarized component in this example) through 90.degree. upon application of a voltage to a liquid crystal layer in accordance with the corresponding color image signal.
In a liquid crystal projection display apparatus having an arrangement described above, only the S-polarized component of the white light emitted from the light source 82 is reflected perpendicularly to the left of FIG. 1 by the polarizing beam splitter 83. The white light of reflected S-polarized light is separated into red light, green light, and blue light by the first and second dichroic mirrors 84 and 85. These light beams are then incident on the corresponding liquid crystal panels 81R, 81G, and 81B.
The polarization planes of the image light beams of the respective colors from the liquid crystal panels 81R, 81G, and 81B are rotated in accordance with image signals of the respective pixels and, therefore, the output image light includes S- and P-polarized light components. The light beams of the respective colors are synthesized by the first and second dichroic mirrors 84 and 85, and the synthesized light is incident on the polarizing beam splitter 83.
The P-polarized components of the light beams of the respective colors are transmitted through the polarizing beam splitter 83 and are projected on a screen (not shown) through the projection lens 86. On the other hand, the S-polarized components of the light beams of the respective colors are reflected perpendicularly downward by the polarizing beam splitter 83 and return to the direction of the light source 82. That is, the polarizing beam splitter 83 serves as both a polarizer and an analyzer for the liquid crystal panels 81R, 81G, and 81B. In this projection display apparatus, polarizing plates for eliminating the S-polarized components of the respective color components are not required for the liquid crystal panels 81R, 81G, and 81B.
The above conventional apparatus, however, has the following drawbacks.
A light beam in the blue light wavelength range is taken as an example. The dichroic mirrors 84 and 85 serving as the color separating/synthesizing means for performing color separation and synthesis of white light have a wider reflected S-polarized component wavelength range than the reflected P-polarized component wavelength range and therefore their reflection characteristics have polarization dependency, as shown in FIG. 2. As in the liquid crystal projection display apparatus shown in FIG. 1, when the light beams incident on the liquid crystal panels 81R, 81G, and 81B include S-polarized components and the image light beams of the respective colors emerging from the liquid crystal panels 81R, 81G, and 81B include P-polarized components, parts of the image light beams of the respective colors are lost due to the polarization dependency of reflection characteristics of the dichroic mirrors 84 and 85. For this reason, brightness and contrast levels are undesirably decreased, and a clear image cannot be projected on the screen.